1. NASA
X-1s RESEARCH RESULTS
ith a Selected Bibliography
4
NATIONAL AERONAUTICS AND SPACE ADMINISTRATION

2. X-15RESEARCH RESULTS
With a Selected Bibliography
By Wendell H. Stillwell
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Scientific and Technical Information Division 1 9 6 5
NATIONAL AERONAUTICS AND SPACE ADMINISTRATION
Washington, D.C.
NASA SP-60

3. For sale by the Superintendent of Documents, U.S. Government
Printing Office,Washington, D.C., 20402-Price 55 cents

4. Foreword
IN A PERIOD of a little more than sixty years since the first
flight of the Wright Brothers, man’s exploration of three-dimen-
sional space above the surface of the Earth has extended beyond the
atmosphere. Spectacular and exciting events in this dramatic quest
have been well publicized. Behind these milestones of practical
flight have been less publicized achievements in scientific research,
making such progress possible. Although the X–15 has had its share
of newsworthy milestones, its contributions to scientific research have
been a more essential and more meaningful part of the program from
its inception. This semi-technical summary of the X–15 program
is directed toward the less publicized aspects of its achievements.
The year 1964 marks the tenth anniversary of the inception of
the X–15 flight-research program, the fifth year since the first X–15
flight. When the program was first approved, its objectives were
clearly stated in terms of aerodynamic heating, speed, altitude, sta-
bility-and-control research, and bioastronautics. Although these
objectives have been essentially accomplished, it now appears that
the three X–15’s may be flown for perhaps another five years, in a
new role as test beds for fresh experiments utilizing the X–15 per-
formance, which still offers more than twice the speed and three times
the altitude capability of any other aircraft now in existence.
Even though the program has been most successful in terms of
achieving its planned objectives and is continuing to play an impor-
tant role in aerospace research, many notable benefits have been of
a different nature—more intangible and somewhat unforeseen at
the time the X–15 program was approved. In the early years of our
nation’s space program, which has been based to a large extent on
the unmanned-missile technology that had been developed over the
five years prior to Project Mercury, the X–15 has kept in proper
perspective the role of the pilot in future manned space programs.
It has pointed the way to simplified operational concepts that should
iii

5. provide a high degree of redundancy and increased chance of suc-
cess in these future missions. All of the people in industry and in
government who have had to face the problems of design and of
building the hardware and making it work have gained experience
of great value to the more recent programs now reaching flight
phase and to future aeronautical and space endeavors of this country.
The X–15 program and Project Mercury have represented a par-
allel, two-pronged approach to solving some of the problems of
manned space flight. While Mercury was demonstrating man’s
capability to function effectively in space, the X–15 was demon-
strating man’s ability to control a high-performance vehicle in a
near-space environment. At the same time, considerable new knowl-
edge was obtained on the techniques and problems associated with
lifting reentry.
Already the lessons learned are being applied to our new manned
space programs. The pilot is playing a much greater role in these
programs. Certainly the problem of launching the lunar-excursion
module from the surface of the Moon through the sole efforts of its
two-man crew must appear more practical and feasible in the light
of the repeated launchings of the X–15 through the efforts of its
pilot and the launch operator on the carrier B–52 than would be
the case if it were compared only with the elaborate launch proce-
dures and large numbers of people, buried safely in blockhouses, that
typify all other launch operations to date. Future space programs
may well include a lifting reentry and a more conventional landing
on Earth, in the fashion demonstrated by the X–15.
Edwards, California Paul F. Bikle, Director
November 1, 1964 NASA Flight Research Center
X–15 RESEARCH RESULTS
iv

6. Contents
CHAPTER PAGE
Foreword iii
Chronology vi
1 The Role of the X–15 1
2 The First Hypersonic Airplane 9
3 Developing a Concept 17
4 Flight Research 33
5 Aerodynamic Characteristics of Supersonic-
Hypersonic Flight 47
6 A Hypersonic Structure 59
7 The Dynamics of Flight 71
8 Man-Machine Integration 83
9 A Flying Laboratory 95
Bibliography 103
Index 117

7. Chronology
June 1952 NACA Committee on
Aerodynamics recommends increase in
research dealing with flight to Mach
10 and to altitudes from 12 to 50 miles.
Also recommends that NACA endeavor
to define problems associated with
space flights at speeds up to the velocity
of escape from Earth’s gravity.
July 1952 NACA Executive Com-
mittee adopts recommendations of
Committee on Aerodynamics.
September 1952 Preliminary stud-
ies of research on space flight and asso-
ciated problems begun.
February 1954 NACA Research
Airplane Projects panel meeting dis-
cusses need for a new research airplane
to study hypersonic and space flight.
March 1954 Laboratories request-
ed to submit views on most important
research objectives and design require-
ments of a new research airplane.
May 1954 NACA teams establish
characteristics of a new research air-
plane, which subsequently becomes the
X–15.
July 1954 Proposal for new research
airplane presented to the Air Force
and Navy.
December 1954 Memorandum of
understanding for a “Joint Project for
a New High-Speed Research Airplane”
signed by representatives of the Air
Force, Navy, and NACA.
December 1954 Invitations issued
by the Air Force to contractors to par-
ticipate in the X–15 design competi-
tion.
September 1955 North American
Aviation, Inc., selected to develop three
X–15 research airplanes.
February 1956 Reaction Motors,
Inc., awarded development contract
for XLR–99 rocket engine.
December 1956 X–15 mock-up com-
pleted.
September 1957 Design configura-
tion set. Construction starts.
October 1958 Factory rollout of No. 1
airplane.
June 8, 1959 First glide flight, No. 1
airplane.
September 17, 1959 First powered
flight, No. 2 airplane.
November 15, 1960 First flight with
XLR–99 engine.
February 7, 1961 Last flight with
interim rocket engine.
March 7, 1961 First flight to Mach 4.
June 23, 1961 First flight to Mach 5.
October 11, 1961 First flight above
200 000 ft.
November 9, 1961 First flight to
Mach 6.
December 20, 1961 First flight of
No. 3 airplane.
July 17, 1962 First flight above
300 000 ft.
November 9, 1962 No. 2 airplane
damaged during emergency landing.
June 27, 1963 50th flight over Mach
4.
January 28, 1964 100th flight in
series.
June 25, 1964 First flight of rebuilt
No. 2 airplane.
August 12, 1964 50th flight over
Mach 5.
August 14, 1964 75th flight over
Mach 4.
October 15, 1964 50th flight by No. 1
airplane; 119th flight in program.
vi

8. CHAPTER 1
The Role of the X–15
N
OT SINCE THE WRIGHT BROTHERS solved the basic
problems of sustained, controlled flight has there been such an
assault upon our atmosphere as during the first years of the space
age. Man extended and speeded up his travels within the vast
ocean of air surrounding the Earth until he achieved flight outside
its confines. This remarkable accomplishment was the culmination
of a long history of effort to harness the force of that air so that he
could explore the three-dimensional ocean of atmosphere in which
he lives. That history had shown him that before he could explore
his ethereal ocean, he must first explore the more restrictive world of
aerodynamic forces.
Knowledge about this world came as man developed theories and
experimental techniques that helped him understand the complex
reaction of air upon a vehicle moving through it. One of the
earliest theories came from Leonardo da Vinci, who sought to explain
the flight of birds. It was Sir Isaac Newton who, among his many
achievements, first put a possible explanation for aerodynamic forces
into mathematical form. Later, crude experiments began to pro-
vide measurable answers to supplement the theories of airflow.
Sometimes the theories failed to stand up in the light of experimental
evidence. Often both theory and experiment gave incomplete
answers.
But man learned to apply this knowledge. Whenever enough
theory was available to answer some questions and enough experi-
mental evidence was at hand to answer others, he has advanced in
flight, often past his full understanding of how he did it. While the
Wright Brothers had learned many answers before their first flight,
men were still trying to discover all the theories that explained it

9. X–15 RESEARCH RESULTS
long after it was history. Every pioneering flight stimulated the
building of other airplanes, and further theoretical and experimental
studies. From all of these efforts has come the detailed under-
standing of the aerodynamics of flight so necessary as the firm foun-
dation upon which aviation progress has been built.
Nowhere is the durability of this foundation more evident than in
the most advanced airplane in the world, the X–15 rocket airplane.
For the mathematical theory that Newton published in 1726—long
discounted because it couldn’t be applied to airflow at low speeds—
is now used to help understand the aerodynamic forces encountered
by the X–15 at speeds of 4000 miles per hour.
The X–15 program is adding to the historic foundation of aero-
dynamics, sometimes measurably, often intangibly, in ways as yet un-
realized. Not only has it doubled the speed of piloted flight; it has
prepared the way for non-orbiting flight into space. It has pushed
piloted flight to an altitude of 67 miles, above 99.999 percent of
the atmospheric ocean. Although the X–15 has provided much
new knowledge about this once-feared region, its return journey from
there has proved even more fruitful. Reentry compounds the effects
of aerodynamic and space flight with a maneuver that is more de-
manding of both pilot and aircraft than any heretofore encountered.
Yet, though severely taxing, reentry flight has been mastered, and
many previous unknowns no longer remain.
Today, after 120 flights, and accumulated flying time of two
hours at speeds above 3000 mph, the three X–15 airplanes show
the effects of having pushed past man’s complete understanding.
Wrinkles and buckles mar the once-sleek fuselages. Gaps have been
cut elsewhere. Scars are visible where the skin of the wings has been
hammered back in place. The three X–15’s appear old and tired
after many pioneering flights. One of them has a vertical tail with
a razor-sharp leading edge, a radical departure from the others.
None of them has the vertical tail with which it first flew. Other
changes are hidden, such as the added structure that stiffens the
fuselage and vertical tail, and the electronics that now help operate
the controls.
The changes came from broad-scale attacks, carried out in three
phases. The first comprised the early flights, which explored the
boundaries of the major research areas. The second consisted of
methodical flights to fill in necessary details. Most of this is now

10. THE ROLE OF THE X–15
Three views of the X–15’s original configuration, with which it achieved a maximum
speed of Mach 6.06 and a maximum altitude of 354 200 ft. Its launch weight
was 33 000 lb.; landing weight, 14 700 lb. The lower half of its vertical tail had
to be jettisoned before landing, since, as the little head-on view makes clear, it
otherwise would have protruded below the landing gear when the latter was
extended.
Hydrogen
peroxide
YLR-99
Engine Anhydrous ammonia
tank (fuel)
Liquid oxygen
tank (oxidizer)
Liquid nitrogen
Ejection seat
Auxiliary
power units
Attitude
rockets
Hydrogen
peroxide Helium
tanks
060051
This cutaway drawing reveals the volume of tankage needed to give the X–15 its
dazzling propulsion, its pressurization, and its attitude control in space. Liquid-
oxygen capacity, 1003 gal.; anhydrous-ammonia capacity, 1445 gal.

11. X–15 RESEARCH RESULTS
history. In the third, and current, phase the X–15 airplanes are
being used more as research tools than research craft. This new
role includes carrying scientific experiments above the atmosphere-
shrouded Earth into regions no satellite or rocket can usefully explore.
The X–15 also serves as a test bed for new components and sub-
systems, subjecting them to a hypersonic flight environment.
Although not all the results of the program are in yet, many im-
portant questions have been answered, some of the major ones during
design and construction. A structure was developed that has with-
stood repeated flights into a searing airflow that has heated large
areas of the structure to a cherry-red 1300o
F. Sometimes the
structure responded in an unexpected way, because of uneven heat-
ing. Hot spots caused irregular expansion, and those wrinkles and
buckles. But while these effects were dramatically visible, they
were always localized and merely slowed the pace of the flight pro-
gram, never stopped it. From this has come a clearer picture of the
combined effects of stresses from aerodynamic loads and aerodynamic
heating.
It has also shown the interplay between airflow, elastic properties
of the structure, and thermodynamic properties of air. The X–15
is the first airplane to push from supersonic speeds to hypersonic
speeds, where the river of airflow heats leading edges to 1300° F.
It provided the first full-scale hypersonic flight data to researchers
who had been concerned with hypersonic theory but who had been
limited to the cold-flow results of existing ground facilities. Those
cold results had produced little agreement among the several theories
for predicting heat flow into an aircraft structure.
From the X–15 data, researchers discovered that theories and ex-
perimental techniques were considerably in error. This significant
result started detailed measurements and analysis of the airflow near-
est the external skin, trying to find the reason. Although the com-
plete story of heat flow is known only in a general way, available
theories have been modified so as to yield dependable predictions
for it at hypersonic speeds.
In addition, the forces that support, slow down, and stabilize the
X–15 can be reliably calculated. The X–15 data have also shown
that small-scale wind-tunnel tests accurately forecast full-scale aero-
dynamic forces, with but minor exception. This increased research-
ers’ confidence in these experimental tools.

12. THE ROLE OF THE X–15
Significant Help From Flight Simulator
One important contribution of the X–15 program is the develop-
ment of a pilot-controlled flight-simulation device that has greatly
aided research. This device combines aerodynamics with an elec-
tronic computer so as to simulate any flight condition likely to be en-
countered by the X–15. With it, many of the unknowns of con-
trolling the X–15 were explored long before the first flight. The
results were somewhat surprising. The region of early concern,
control in space, was found to contain no serious problems. Yet
the time-honored criteria used to predict aircraft stability had failed
to uncover a major pitfall. The result: without some aid from
electronic control, the X–15 would be uncontrollable over a large
part of the anticipated flight envelope.
This major obstacle was overcome, but not without changes to the
airplane’s tail surfaces and control system as well as to its stability
criteria. Analysis techniques were developed that helped explain
the phenomena. Significantly, automatic control came to be looked
upon not as a replacement for the pilot but as a useful, helpful, even
necessary aid, without which the full potential of the X–15 would
not have been achieved.
In addition to contributing to high-speed flight, the X–15 program
lowered a barrier at the low-speed end of the flight, for the subse-
quent landing. This landing was expected to be critical, since it
would require such precise judgment and control by the pilot that
he would have no margin for error. But techniques were developed
that gave back to the pilot enough margin so that the landing is
now a routine maneuver. Pilots and aerodynamicists now plan with
confidence the landing of future airplanes that will have even more
extreme landing characteristics.
The X–15 pilots removed one earlier barrier, a psychological one.
When some scientists looked spaceward, they became concerned that
man himself would be the limiting factor. Indeed, in the missile
dawn of the early 1950’s, a large segment of the aeronautical industry
began to speculate that man might soon be relegated to pushing
buttons. No one working on the X–15 project agreed with this
view, least of all the pilots. They viewed hypersonic and space flight
as a demanding expansion of previous flight experience, not a radical
departure. Now, 120 flights have shown us that this traditional

13. X–15 RESEARCH RESULTS
concept for piloted flight research, while needing some modification,
is also applicable to the space era. Many now wish that all the X–15
components would exhibit the same steady, competent reliability
that the pilots do.
Perhaps the X–15’s most significant role has been to sustain
interest in manned, maneuverable flight in high-speed aircraft during
a period when the world’s gaze turned to orbital space flight. The
existence of this active program stimulated creative thought and
focused attention on the future of hypersonic aircraft in the rapidly
advancing age of space travel. Now that men have begun long-
range planning of the nation’s space program, they envision daily
shuttle runs to orbital space laboratories and foresee the need for
efficient, reusable space ferries to cross the aerodynamic river. Scien-
tists now talk of two-stage rocket planes and recoverable boosters.
Also proponents of the two principal means for orbital and super-
orbital reentry—ballistic capsule and lifting body—are coming closer
together, for the force that brakes a capsule can be utilized for
maneuvering, as the X–15 has proved. Although the stubby wings
of the X–15 may look rather puny, many space officials believe they
point the way to the future. Thus the X–15 and Mercury programs
are seen, in retrospect, as having made a valuable two-pronged con-
tribution to future manned space flight.
Many strong building blocks have come from the experience of
doing-the-job; from learning safe operational techniques and flight
procedures; from gaining experience with piloted hypersonic flight
and non-orbiting space flight as well as with the intricacies of mis-
sile-type operations with large rocket engines and a two-stage aero-
space-booster configuration. This is knowledge that may someday
pay off in unexpected ways.
But if the X–15 program has been the source of much new knowl-
edge, it is because there were many unknowns when this bold
program was undertaken. A large measure of the success of the
program is due to the individuals of extraordinary vision who had
the resolution to push ahead of these unknowns. They were men
who were prepared to take giant steps, sometimes falteringly, not
always successfully, but eventually yielding results. They were men
who knew that the foundations upon which the X–15 would be
built were sound, yet knew they couldn’t wait for all the answers
before going ahead. They knew that to go ahead with incomplete

14. THE ROLE OF THE X–15
knowledge would invite failure, and that technological barriers can
become psychological barriers as well. They had no intention of
trying to batter down these barriers. They knew that measurable
contributions would come from studying and probing until enough
unknowns were removed so that they could ease their way through
to the next obstacle. For they had their sights set way ahead of the
X–15, to its successor, and another.
This diagram shows how the X–15 has explored the aerodynamic-flight corridor to
a speed of 4000 mph and the space-equivalent region above it to an altitude of
67 miles. The aerodynamic-flight corridor is the pathway that reentry spacecraft
of the lifting-body type would use to return to Earth from orbital space stations.

15. X–15 RESEARCH RESULTS
From their stimulus, the United States acted, and acted fast.
Initiated as a matter of national urgency, the program emerged from
behind security restrictions to become intimately associated with
national prestige. Today, a successor is still many years away, and
the X–15 remains the only aircraft capable of studying phenomena
at hypersonic speeds, space-equivalent flight, and reentry flight.
And it has gained a new role as a workhorse. Rarely has a research
program encompassed so many fields of basic and applied science,
and less often still has any been able to contribute for such a long
period in a fast-advancing technological age. Yet, just as the Wright
Brothers left many questions unanswered, today, long after the X–15
first flew 4000 mph, men are still trying to find a complete explana-
tion for airflow. But as long as Earth’s atmosphere exists, whenever
men fly that fast, they will be traveling in a region whose secrets
the X–15 was first to probe.

16. CHAPTER
2
The First Hypersonic Airplane
THROUGH TWO HUNDRED YEARS of analysis and experi-
ment, scientists and engineers have slowly accumulated a detailed
picture of flight through our atmosphere. They know that at high
speeds the dense layer of air close to the Earth’s surface generates
pressures that hinder an aircraft, while at high altitudes the air density
is so low that extremely fast speeds are necessary to generate enough
pressure to keep a plane flying. They designed airplanes as a com-
promise between these forces, and flight became confined to a corri-
dor that is bounded by ever-increasing combinations of altitude and
velocity.
As man pushed aircraft farther up this flight corridor, the problems
began to multiply. New aerodynamic knowledge and new scientific
disciplines had to be added to the world of airflow. The concept of
the atmosphere as a single gaseous envelope gave way to one that
recognized it as a series of layers, each with its own characteristics.
Airflow, too, was found to have distinct regions and characteristics.
At velocities less than 500 mph, it is tractable and easily defined. At
higher speeds, its character undergoes marked change, sometimes
producing abrupt discontinuities in aerodynamic pressures. Even
before man’s first flight, the noted German physicist Ernst Mach had
shown that a major discontinuity occurs when the velocity of airflow
around an object approaches the speed of sound in air (760 mph at
sea-level pressure and temperature). Later work showed that the
air pressures an airplane experiences vary with the ratio of velocity
of airflow to speed of sound, and scientists adopted this ratio, called
Mach number, as a measure of the flow conditions at high speeds.
The effect of flight to Mach 1 produces large changes in the air
pressures that support, retard, twist, pitch, roll, and yaw an airplane.
But man edged past this speed into the realm of supersonic flight,
and by the time Mach 1.5 was attained, airplanes had undergone a
vast transition in technology. Some men saw in this transition

17. X–15 RESEARCH RESULTS
10
the basis for pushing much farther up the flight corridor. In the
early 1950’s, a few visionary men looked far up that corridor and
became intrigued by a goal much closer than the theoretical limit at
the speed of light. They saw that the corridor flared dramatically
upward at orbital speed (Mach 24), leading out of the Earth’s at-
mosphere into space, defining the start of a path to the Moon, Mars,
and beyond.
But if their gaze was on orbital flight, their minds were on a torrent
of new problems that had to be overcome to achieve it. The super-
sonic-flight region led into hypersonic flight—a fearsome region with
a thermal barrier, which looked far more formidable than had the
earlier, sonic barrier. This new barrier came from the friction of air
as it flows around an aircraft. At Mach 10, that friction would
make the air hot enough to melt the toughest steel. At Mach 20,
the air temperature would reach an unbelievable 17 000o
F. Thus
aerodynamic heating was added to the growing list of new disciplines.
Other new problems came into view. Flight above the atmos-
phere would render aerodynamic controls useless, requiring another
method of control. The pilot’s response to the weightlessness of
orbital flight was a controversial subject. Some expressed grave
doubts that he could withstand prolonged periods of orbital flight.
The reentry into the atmosphere from space would perhaps com-
pound all of the problems of hypersonic flight and space flight. Yet
these problems were academic unless powerplants an order of magni-
tude more muscular than were then available could be developed to
propel an aircraft into space. Little wonder, therefore, that the
pioneers envisioned a slow and tortuous route to reach their goal.
They had yet to realize that manned orbital flight was possible in one
big jump, through the wedding of large ballistic missiles and blunt
reentry capsules.
The vision of these men, however, began to stimulate thought
and focus interest within the aeronautical community on the pros-
pects for orbital flight. Early studies showed that much could be
learned about space flight without achieving orbital speeds. By
zooming above the normal flight corridor at less than orbital speeds,
one could study non-aerodynamic control and weightlessness. Re-
entry from such a maneuver would approximate reentry from space.
Perhaps more significant was the fact that if a speed of Mach 8–10
could be achieved, aerodynamics would be over the hump of hyper-

18. THE FIRST HYPERSONIC AIRPLANE 11
sonic flow, for air pressures show far less variation above this speed
than below.
The initial investigative work was guided by extensive theoretical
analysis and ground-facility experiments, but critical problems
abounded and possible solutions were largely speculative. Theo-
retical methods approximated an airplane as a cone and cylinder,
with wings composed of flat plates. While these theories agreed
with some of the results of wind-tunnel experiments, there were
many disagreements. There were doubts about the accuracy of
wind-tunnel measurements, because of their extremely small scale.
Although large hypersonic tunnels were being developed, an airplane
had already flown faster than the top speed that could be duplicated
in any wind tunnel big enough for reliable development-testing.
Many of the pioneers became convinced that the best way to attack
the many unknowns would be to meet them head-on­—in full-scale
flight research. They pressed for an airplane to make the first step
into the hypersonic, space-equivalent, and reentry flight regimes,
to lay the groundwork for following airplanes. A decisive influence
was the fact that rapid progress was already being made on the
development of powerful, liquid-fueled rocket engines, though they
were not intended for airplanes.
Among the several visionary men of the era, the late Robert J.
Woods, of Bell Aircraft Corp. (now Bell Aerospace Corp.), was
outstanding. His efforts to “sell” manned space flight began in
June, 1952, some five years before the Earth’s first artificial satellite
appeared. In a bold proposal, he urged the United States to “evalu-
ate and analyze the basic problems of space flight . . . and endeavor
to establish a concept of a suitable test vehicle.” One important
and, to Woods, fundamental part of his recommendation was that
the (then) National Advisory Committee for Aeronautics should
carry forward this project. NACA was a government organization
(later forming the nucleus of the National Aeronautics and Space
Administration) that had long been in the forefront of high-speed
aeronautical research. Many of the foremost proponents of hyper-
sonic flight were on its staff. NACA had also coordinated aero-
nautical technology among the military services, civil aviation, and
aircraft industry, and was responsive to their respective needs.
NACA was most active and eager for a bold step into hypersonic
flight.

19. X–15 RESEARCH RESULTS
12
Basic Studies Began in 1954
But at a time when the current struggle was to push aircraft speeds
from Mach 1.5 to 2.0, two more years elapsed before a climate de-
veloped in which the urgency for hypersonic flight was backed up
by resources of money and manpower. In March, 1954, NACA’s
Langley Aeronautical Laboratory, Ames Aeronautical Laboratory,
and High Speed Flight Station began the studies that led to the X–15
program. This early work was the first to identify all major pro-
blems in detail and examine feasible solutions. Only then could
the researchers decide how big their first step should be.
They knew at once that Mach 8–10 was unobtainable. Materials
and technology were not available for such speeds. But the work of
the Langley Laboratory showed that Mach 6–7 was within reach,
as well as an altitude of 250 000 feet, well above the conventional
flight corridor. And, of course, even Mach 6 was a giant step. To
attain this speed would require a rocket engine of 50000-pounds
thrust and a weight of propellants 1½ times the weight of the basic
airplane. These were difficult goals, but within the state of the
art.
The major problems would be to achieve a configuration that
was stable and controllable over the entire range of speed and alti-
tude, and prevent it from being destroyed by aerodynamic heating.
The stability-and-control problem appeared to be solvable, although
a few innovations would be required. Most importantly, the Lang-
ley study pointed to a way through the thermal barrier. It showed
that if the airplane were exposed to high-temperature airflow for
only a brief period of time, its structure could be designed to absorb
most of the heating, and temperatures could be restricted to a maxi-
mum of about 1200o
F. This concept of a “heat sink” structure
was based upon use of a new high-temperature nickel-chrome alloy,
called Inconel X by its developer, the International Nickel Co.
Inconel X would retain most of its strength at 1200o
F, a temperature
that would melt aluminum and render stainless steel useless. How-
ever, no manufacturer had ever made an aircraft of Inconel X.
The Langley study influenced the X–15 program also through
its somewhat philosophical approach to the craft’s development
and method of operation. In the view of the Langley study team,
any new airplane should be a flight-research tool to obtain a maxi-

20. THE FIRST HYPERSONIC AIRPLANE 13
mum amount of data for the development of following airplanes.
The design, therefore, should not be optimized for a specific mis-
sion, but made as useful as possible for exploratory flight—a rather
vague criterion. A tentative time limit of only three years was
set for the design and construction, in order that flight data could
be obtained as soon as possible. Such a tight schedule established
the need for somewhat of a brute-force approach. The design
must stay within the state of the art and avoid the use of uncon-
ventional techniques that would require long development time.
Other Langley guidelines specified the use of proven techniques
as far as possible, and “the simplest way to do the job.” They em-
phasized that the airplane should not become encumbered with
systems or components not essential to flight research. These re-
quirements were tempered by knowledge that a three-year develop-
ment schedule would leave little or no time to perfect systems and
subsystems before first flight.
The design philosophy was also influenced by the fact that new
aerodynamic regimes were to be explored in a carefully regulated,
progressive manner, thus gradually exposing the airplane and pilot
to any critical condition for which complete data might have been
impossible to obtain during the speeded-up design period. Signifi-
cantly, early plans were for the flight program to be conducted by
NACA’s High Speed Flight Station (now NASA’s Flight Research
Center) at Edwards, California, which at that time functioned as a
part of the Langley Laboratory at Hampton, Virginia, though sepa-
rated from it by some 2300 miles. This close tie brought into the
program at the very beginning the viewpoints of the research pilots
who would fly the X–15.
An important figure in the over-all coordination was H. A.
Soulé, of the Langley Laboratory, who had directed NACA’s part
in the research-airplane program since 1944. He and his chief
associates would steer the X–15 program through the conceptual
studies and the design and construction phases with one goal: to
develop a satisfactory airplane in the shortest practical time. This
meant severe pruning of a multitude of proposed engineering studies,
every one of which could be justified in the cause of optimization, but
which together could lead to fatal over-engineering in the effort to
achieve an ideal aircraft. It also meant stern attention to the prog-
ress of selected studies. Mr. Soulé’s task was complicated by the

21. X–15 RESEARCH RESULTS
14
fact that the interests of other government organizations would have
to be served at the same time, since NACA’s resources were too
meager to enable it to undertake such an ambitious program alone.
By the fall of 1954, a technical proposal and operational plan had
been formulated and presented to several government-industry ad-
visory groups on aviation. NACA proposed that the new program
should be an extension of the existing, cooperative Air Force-Navy-
NACA research-airplane program. This joint program, which
dates from 1944, had resulted in the well-known first flight to Mach
1, by the X–1 rocket airplane; the first flight to Mach 2, by the
D–558–II rocket airplane; and the first flight to Mach 3, by the
X–2 rocket airplane. Less well-known are 355 other rocket-air-
plane flights and more than 200 jet-airplane flights made under this
program. These were flights that in 1947 helped lay bare some of
the problems of transonic flight, at speeds now commonplace for
jet transports. These flights also laid the technical and managerial
foundations for the X–15 program, and led to its immediate and full
support by the United States Air Force, Navy, and Department of
Defense.
Because of the magnitude of the new research-airplane program,
a formal Memorandum of Understanding was drawn up among
the Air Force, Navy, and NACA, setting the basic guidelines upon
which the program operates to this day. A distinctive feature of the
memorandum is that it is not just a definition of the lines of authority
and control. Rather, it lays out a fundamental pattern of coopera-
tion among government agencies that continues as a basic feature
of the X–15 program, and has had no small effect on the successful
pursuit of the research. In essence, it states briefly that each partner
agrees to carry out the task it is best qualified for.
The Memorandum of Understanding may also be the only place
where the true purpose of the X–15 program is spelled out. This is
contained in a specific provision for disseminating the results of the
program to the U.S. aircraft industry. It adds that the program is
a matter of national urgency.
This urgency was already obvious. In less than 10 months from
the time NACA initiated the study to determine if hypersonic flight
was feasible, a detailed program had been submitted to the aircraft
industry, and several firms were already making preliminary design
studies for flight to Mach 6–7. This rapid progress, perhaps more

22. THE FIRST HYPERSONIC AIRPLANE 15
Noted predecessors of the X–15 in the cooperative research-airplane program of the
Air Force, Navy, and NACA, dating from 1944, were the X–1 (above), which made
the world’s first supersonic flight, and the X–2 (below), which first flew to Mach 3.

23. X–15 RESEARCH RESULTS
16
than any other factor, tells of the invisible pressure that had resulted
from the stimulus of the strong individuals who pioneered the X–15.
A national program to develop the world’s first hypersonic airplane
was underway.

24. 17
CHAPTER 3
Developing a Concept
W
HEREAS THE COMPLETE DEVELOPMENT of the first
powered aircraft was carried out by two men, the complexities
of a modern aircraft require a ponderous procedure to shepherd it
from technical proposal through design and construction and to
provide support during its flight program. For the X–15, this shep-
herding role has been provided by the Aeronautical Systems Division
(formerly Wright Air Development Center) of the Air Force Systems
Command. It has included research-and-development support not
only for an airplane with revolutionary performance but also for the
most powerful—and potentially most dangerous—powerplant ever
developed for aircraft use. lt has encompassed new concepts for
pilot protection, numerous first-time subsystems, modifications and
support for two launch airplanes, and the eventual rebuilding of two
of the original three X–15’s. It will surely include other items as
the program goes on.
A third partner joined the X–15 team when North American Avia-
tion, Inc., won the design competition with other aircraft manu-
facturers. The proposal of the Los Angeles Division of NAA was
chosen by joint Air Force-Navy-NACA agreement as “the one most
suitable for research and potentially the simplest to make safe for the
mission.” The contract called for the construction of three aircraft,
with the expectation that two would always be in readiness and one
undergoing modification or repair. Two craft would have been
enough to handle the anticipated research workload, but if there had
been only two, a mishap to one of them—always a strong possibility
in exploratory flight research—would have seriously curtailed the
program.
Although NACA’s studies showed possible solutions for many of
the major problems, it remained for one of America’s crack design
groups actually to solve them. And it is also true in any ambitious

25. X–15 RESEARCH RESULTS
18
endeavor that the magnitude of a problem seldom becomes fully ap-
parent until someone tries to solve it. The basic problem North
American faced was that of building an airplane of new materials to
explore flight conditions that were not precisely defined and for which
incomplete aerodynamic information was available. Yet it would
have to accomplish this on an abbreviated schedule, despite an ap-
palling inadequacy of data.
The original design goals were Mach 6.6 and 250 000 feet, but
there were no restrictions to prevent flights that might exceed those
goals. The flight program would explore all of the corridor to the
maximum practical speed, and it would investigate the space-equiva-
lent region above the corridor. The reentry maneuver would com-
pound many factors. Both airplane and pilot would be subjected
to acceleration forces of six times gravity (6 G’s). The pilot would
be required to maintain precise control during this period, with both
airplane and control system undergoing rapid, large changes in
response. These nice generalities had to be translated into hard,
cold criteria and design data.
The development of any aircraft requires many compromises,
since a designer seldom has a complete answer for every problem. If
there are unlimited funds available to attack problems, an airplane
can represent a high degree of perfection. But this process is also
time-consuming. For the X–15 design, compromises were all the
more inevitable and the optimization difficult. The X–15 would
still be on the drawing boards if construction had been delayed until
an ideal solution to every problem had been found. A reconcilia-
tion of the differing viewpoints of the several partners in the program
was also necessary. While all were agreed on the importance of the
program, their diverse backgrounds gave each a different objective.
In spite of differences, the project rolled ahead. In all of this,
the overriding consideration was the brief time schedule. There
literally was no room for prolonged study or debate. While three
years may seem at first to be ample time in which to produce a new
airplane, it must be remembered that simpler aircraft than the X–15
normally require longer than that for construction. For a new flight
regime and use of a new structural material, the X–15 schedule
was most ambitious. One important help in meeting it, however,
was the initial decision to explore new flight regimes in a progressive
manner, so that complete solutions for every problem need not be

26. DEVELOPING A CONCEPT 19
found before the first flight. Not all could be put off until the
flight program began, though.
The sum product of a year of study, a year of design, and a year
of construction is an airplane that is a composite of theory, wind-tun-
nel experiments, practical experience, and intuition­—none of which
provides an exact answer. The X–15 represents an optimization,
within limits, for heating, structure, propulsion, and stability. It
is also a compromise, with many obvious and not so obvious depar-
tures from previous jet- or rocket-plane experience. The fuselage
consists largely of two cylindrical tanks for rocket-engine propellants.
To these were added a small compartment at the forward end, for
the pilot and instrumentation, and another at the aft end, for the
rocket engine. Large, bulbous fairings extend along the sides of the
fuselage to house control cables, hydraulic lines, propellant lines, and
wiring that has to be routed outside the tanks. The big fuselage-
An X–15 is seen just prior to launch from a B–52 at 45 000 ft., 200 miles from home
base. The X–15 pilot und B–52 launch-panel operator have completed their pre-
launch check procedures. The chase plane in the distance is keeping a sharp
eye on the X–15 during the checkout.

27. X–15 RESEARCH RESULTS
20
fairings combination has a decided effect on the total aerodynamic
lift of the X–15. The airflow near these surfaces provides well over
half of the total lifting force, particularly at hypersonic speeds. Thus,
the small size of the wings reflects the relatively small percentage of
lifting force they are required to provide. (They do most of their
work during launch and landing.)
The design of the structure to withstand hypersonic flight brought
one of the prime purposes of the X–15 into sharp focus: to gain
knowledge about heating and the hot structural concept. The struc-
ture could have been protected by insulation or cooling techniques
that would have kept temperatures well below 1200o
F. A basic
feature of the X–15 concept, however, was that a hot structure
would permit more to be learned about aerodynamic heating and
elastic effects than one protected from high temperatures. There-
fore, 1200o
F became a goal rather than a limit.
The complicating factor was that loads and temperatures must
be absorbed with a minimum of structural weight. Yet Inconel X
weighs three times as much as aluminum, and any excess weight has
a critical effect on the performance of a rocket airplane. Each 500
pounds cuts performance by 100 mph, and structural engineers
strive to shave every ounce of extra material from the structure.
An instant or two after launch, the X–15 is seen roaring off on its own, with Inconel
X skin glistening in the sun. At burnout, it will be accelerating at 4 G’s, or 90
additional miles per hour every second.

28. DEVELOPING A CONCEPT 21
This science, or art, had already advanced aluminum aircraft
structures to a high level of load-carrying efficiency. There were
always unpredictable, troublesome interactions, however, and struc-
tural designers usually relied upon laboratory tests to confirm each
new design. Normal practice was to build one airplane that was
statically loaded to the equivalent of anticipated flight loads, in order
to evaluate its strength.
The X–15, however, would have to enter the high-load region
without this time-honored test of its structure. The most severe
stresses are encountered when the structure undergoes aerodynamic
heating, and no static-test facility existed in which the X–15 could
try out a realistic temperature environment. Therefore, a static-test
airplane was not built, and no tests were made of actual structural
components. But the structural design for the high-temperature
condition wasn’t left to analysis alone. An extensive testing pro-
gram was conducted during the design to prove out the approaches
being taken. Many tests were made of sections of the structure
under high temperatures and thermal gradients. These helped de-
fine some of the difficulties and also improved static-test techniques.
NAA saved considerable weight through the use of titanium in
parts of the internal structure not subject to high temperatures.
Titanium, while usable to only about 800o
F, weighs considerably
less than Inconel X. The structural design was influenced to some
extent by the requirements for processing and fabricating these ma-
terials. Inconel X soon stopped being just a laboratory curiosity.
Production-manufacturing techniques were developed to form,
machine, and heat-treat it. In many instances, an exhaustive de-
velopment program was required just to establish the method for
making a part. Thus much practical experience was gained in the
design, fabrication, and testing of new materials.
Additional weight was saved on the X–15 by the use of a rather
novel landing-gear arrangement. The main landing gear consists
of two narrow skis, attached at the aft end of the fuselage and stowed
externally along the side fairings during flight. When unlocked,
the skis fall into the down position, with some help from airflow.
A conventional dual-wheel nose gear is used. This gear is stowed
internally to protect its rubber tires from aerodynamic heating.
Contrasting with the X–15’s small wings are its relatively large
and massive tail surfaces. These surfaces, like the fins on an arrow,

29. X–15 RESEARCH RESULTS
22
stabilize the craft in its flight. But, unlike an arrow, which ideally
never veers from its path, the X–15 must be able to change align-
ment with the airflow, to maneuver and turn. And it is a most
difficult design compromise to achieve the proper balance between
stability and control. The problem in this case was greatly com-
plicated by the different aerodynamic-flow conditions encountered
within the flight corridor, and by the changes between the angle of
airflow and the pitch and yaw axes required to maneuver the air-
plane. Criteria had been developed to guide the design, but they
were derived largely from empirical data. They required consid-
erable extrapolation for X–15 flight conditions.
Although the extrapolation in speed was rather large, the largest
was in the extreme angle between pitch axis and flight path re-
quired for pullout during reentry. This results in a compounding
problem, because it becomes increasingly difficult to stabilize an air-
plane at high angles of airflow (angle of attack) at high speed. A
This photograph provides an unusually clear view of the X–15’s unique main land-
ing gear, and shows how much of the lower vertical tail is left after its bottom
part has been jettisoned for landing. The photo also reveals the new, knife-sharp
leading edge that has been given to the X–15–3 configuration’s upper vertical tail
in order to study heat transfer through 2500-deg. airflow.

30. DEVELOPING A CONCEPT 23
phenomenon is encountered in which the vertical tail loses ability
to stabilize the airplane and the nose tends to yaw. Indeed, the only
previous airplanes that had been flown to Mach numbers above 2—
the X–1A and X–2—had experienced such large decreases in sta-
bility that the pilots lost control (disastrously, in the case of the X–2)
when they maneuvered the craft to angles of attack of only 5 or 6
degrees. Yet the reentry maneuver of the X–15 would normally
require it to operate at an angle of attack of 20 to 25 degrees.
The initial solution, proposed by NACA, was found in the large,
wedge-shaped upper-and-lower vertical-tail surfaces, which are
nearly symmetrical about the aft fuselage. A wedge shape was used
because it is more effective than the conventional tail as a stabilizing
surface at hypersonic speeds. A vertical-tail area equal to 60 per-
cent of the wing area was required to give the X–15 adequate direc-
tional stability. Even this was a compromise, though, for weight and
different flight conditions. As an additional factor of safety, there-
fore, panels that could be extended outward, thus increasing the
pressure and stabilizing forces, were incorporated in the vertical
tails. These panels—another NACA proposal—also serve as speed
brakes, and the pilot can use them at any time during flight. Both
braking effect and stability can be varied through wide ranges by
extension of the speed brakes and by variable deflection of the tail
surfaces. The large size of the lower vertical tail required for ade-
quate control at high angles of attack required provision for jet-
tisoning a portion of it prior to landing, since it extends below the
landing gear.
A disadvantage of the wedge shape is high drag, caused by airflow
around its blunt aft end. This drag force, when added to the drag
from the blunt aft ends of the side fairings and rocket-engine nozzle,
equals the entire aerodynamic drag of an F–104 jet fighter.
Control for maneuvering flight is provided by partially rotatable
horizontal-tail surfaces. Roll control is achieved by a unique mecha-
nism that provides differential deflection of the left and right hori-
zontal-tail surfaces. This somewhat unconventional control tech-
nique, called “rolling tail,” was unproven at the time of the X–15
design. However, NAA had studied such control systems for several
years, its studies including wind-tunnel experiments from subsonic to
supersonic speeds. Pitch control is provided by deflecting the left
and right horizontal tails symmetrically.

31. X–15 RESEARCH RESULTS
24
The combination of large control surfaces and high aerodynamic
pressures forced designers to use hydraulic systems to actuate the
surfaces. This type of power steering introduces its own character-
istics into aircraft-control response, as well as making the airplane
The blunt aft ends of the X–15’s side fairings, vertical tails, and the rocket-engine
nozzle represent one of the many compromises that a hypersonic configuration
demanded.Together they produce as much drag as an F–104 jet fighter.

32. DEVELOPING A CONCEPT 25
absolutely dependent upon the proper functioning of the hydraulic
system. It did facilitate the incorporation of electronic controls,
which were shown to be helpful to the pilot, especially during reentry.
There had to be assurance, however, that a malfunction of any com-
ponent during flight would not introduce unwanted control motions.
Thus the design of the control system provided a safe alternative
response in the event of any component failure as well as for normal
operation. Many of the X–15’s operating characteristics are simi-
larly based upon fail-safe considerations.
A unique feature of the control system is the three control sticks
in the cockpit. One is a conventional center stick, which controls
the airplane in pitch and roll as it would in a jet fighter or a Piper
Cub. The center stick is directly linked to one that is at the pilot’s
right side. The latter is operated by hand movement only, so the
pilot’s arm can remain fixed during high accelerations experienced
The X-15’s cockpit is quite like a jet fighter’s, except for its unique arrangement of
three control sticks. The one at left governs the jet reaction controls, in space-
equivalent flight. The one at right is used in high-G flight and is mechanically
linked to the conventional stick at center.

33. X–15 RESEARCH RESULTS
26
during powered flight and reentry. This is an essential feature,
which enables the pilot to maintain precise control for these condi-
tions. The third control stick is located at the pilot’s left, and is
used to control the X–15 when it is above the atmosphere. This
stick actuates reaction jets, which utilize man’s oldest harnessed-
energy form, steam. The X–15 uses a modern form of superheated
steam, from the decomposition of hydrogen peroxide (H2
O2
). This
concept was later adopted for the Mercury-capsule jet controls.
The reaction thrust is produced by small rocket motors located in the
nose, for pitch and yaw control, and within the wings, for roll con-
trol. While such a system was simple in principle, control by means
of reaction jets was as novel in 1956, when it was introduced, as
orbital rendezvous is today. The transition from aerodynamic con-
trol to jet control loomed as the most difficult problem for this vast,
unexplored flight regime.
There were many other new and peculiar conditions for the pilots
to face. Altogether, they would be tackling the most demanding
task ever encountered in piloted aircraft. Some of the control-
system and physical characteristics were tailored to their capabilities
to attain the desired airplane-pilot combination. While the pilot
is an integral part of the concept, with maximum provision made for
his safety, he needs to be able to escape from unforeseen hazardous
conditions. The difficulty, in the case of the X–15, was that to
create a system that would protect the pilot during escape anywhere
within the flight corridor or above it would require a development
program nearly as large as that of the airplane. lt would also re-
quire a prohibitive increase in airplane weight. The result was that
an over-all escape capability was not provided. The airplane itself
was regarded as the best protective device for the pilot at high speeds.
At low speeds, he could use an ejection seat similar to that used in
most military aircraft.
But “low speed” for the X–15 is 2000 mph, and to provide for
escape over this much of the corridor required a state-of-the-art
advance in escape systems. Extensive wind-tunnel and rocket-sled
esting was necessary to achieve an aerodynamically stable ejection
seat. Another major effort was required to provide protection for
the pilot against windblast during ejection. Finally, the desired
escape capability was provided by a combination of pressure suit
and ejection seat.

34. DEVELOPING A CONCEPT 27
Major Advance in Powerplant Needed
Aircraft speeds couldn’t be pushed far up the flight corridor with-
out major advances in powerplants. And the farther up the cor-
ridor one goes, the tougher the going gets. There’s enough power
in one engine of the trusty old DC–3 to pull a planeload of passen-
gers along at 100 mph, but it isn’t enough even to pump the propel-
lants to the X–15 rocket engine. Although by 1955 the United
States had eight years’ experience with aircraft rocket engines, one
of 50 000-pounds thrust was a big advance over any used for that
purpose before. Missiles had provided the only previous experience
with large rocket engines. And the X–15 couldn’t become a one-
shot operation. Its engine would have to be an aircraft engine,
capable of variable thrust over at least 50 percent of the thrust range
and having other normal cockpit control features, such as restarting.
A major problem was the threat of a launch-pad disaster with such
a large rocket engine and the enormous amount of fuel carried
aboard the X–15. This potential danger had to be minimized not
only to insure the safety of the X–15’s pilot but that of the pilot and
crew of the B–52 that would launch it. Thus, safety of operation
became an overriding consideration for the X–15 engine.
A closeup of the X–15’s remarkable XLR–99 rocket engine. Its 57 000-lb. maximum
thrust is equivalent at burnout to 600 000 hp. The engine can be throttled from
40-percent to 100-percent thrust. Its propellants flow at the rate of 13 000 lb. per
minute at maximum thrust, exhausting the entire 18 000-1b. fuel supply in 85
seconds. The engine’s nozzle diameter is 39.3 in.; its over-all length, 82 in.; its
weight, 1025 lb.

35. X–15 RESEARCH RESULTS
28
The problem was two-fold. The huge amount of fuel that was
pumped through the engine meant that in the event of engine mal-
function, a lot of unburned fuel could accumulate in a fraction of a
second. At the same time, combustion difficulties were inherent in
an engine in which burning takes place by mixing two liquids to-
gether, rather than a liquid and a gas, as in a jet engine or auto-
mobile engine. While initially it appeared that a missile engine
could be adapted for the X–15, it soon became evident that none
would meet the stringent safety requirements.
Subsequently, Reaction Motors, Inc. (now the Reaction Motors
Division of the Thiokol Chemical Corp.), was selected to develop
what became the XLR–99 rocket engine. lt was clear that this
firm was undertaking the development of more than just a suitable
engine composed of thrust chamber, pumps, and controls. The
technical requirements contained a new specification that “any single
malfunction in either engine or propulsion system should not create
a condition which would be hazardous to the pilot.”
Reaction Motors was well prepared for this task. lt had built
many rocket engines for the X–1 and D–558–II research airplanes,
and in some 384 flights it had never had a disastrous engine failure.
As a result of this background, its engineers adopted a rigorous de-
sign philosophy that left its mark on every detail part in the propul-
sion system. While endeavoring to prevent malfunctions, they de-
signed the engine so that the conditions following any malfunctions
would be controlled before they became hazardous. They ac-
complished this by developing an igniter system that insures that all
residual propellants are vaporized and burned in the combustion
chamber. Another feature was a system that automatically moni-
tors engine operation and senses component malfunctions. When-
ever a malfunction occurs, the system shuts down the engine safely.
For some controls, component redundancy was used to provide
safety against a single malfunction. However, rather than parallel
entire components, some unique designs were developed that utilize
redundant paths within components.
The added complexities needed to achieve safe operation of the
X–15 engine make it appear to be a “plumber’s nightmare” when
compared to other rocket engines of its era. And normally the
penalty for complexity is reduced reliability in operation. But in-
flight reliability has been 96 percent—a remarkable figure com-

36. DEVELOPING A CONCEPT 29
pared to that of missile engines of similar design. lt is obvious that
safety of operation was not gained at the sacrifice of over-all
reliability.
The engine burns a mixture of anhydrous ammonia (NH3
) and
liquefied oxygen (LO2
). These propellants pose a few handling
problems, because of the corrosive properties of ammonia and the
low temperatures of liquid oxygen, which boils at –297o
F. Since
the propellant tanks are an integral part of the airplane structure,
temperature extremes between structure close to the lox tank and
surrounding structure have exerted a major influence on thermal
stresses and structural design. The lox tank has a capacity of 1003
gallons; the ammonia tank, 1445 gallons. This gives a burning
time of 85 seconds at full thrust. An important feature of the X–15’s
lox system is the need for replenishing it after takeoff, because of the
large amount lost through boil-off during the climb to launch alti-
tude aboard the B–52. This topping-off takes place continuously,
under control of a B–52 crewman, from tanks within the B–52,
which have a capacity 1½ times that of the X–15’s.
The X–15 also carries, besides engine propellants, vast quantities
of hydrogen peroxide (H2
O2
), liquefied nitrogen (–310o
F),
gaseous nitrogen, and gaseous helium (–240o
F), used to operate
various subsystems. With such large amounts of super-cold liquids
flowing within the airplane, its internal components need protec-
tion from freezing, not high temperature. This paradoxical situa-
tion in a hot airplane requires the use of many heating elements and
insulation blankets. (Another paradoxical situation is that Inconel
X, of which much of the plane is made, not only resists high heating
but retains excellent material properties at temperatures as low as
–300o
F.)
Many unique systems and subsystems had to be developed to meet
a host of new power requirements and functions. The auxiliary-
power requirements, in particular, were severe, for not only is there
large demand for hydraulic and electrical power but the aerody-
namic controls will not function without hydraulic power. There-
fore, dualization is used in critical components, from fuel tanks to
hydraulic actuators. The hydraulic pump and electrical generator
are driven by a 50 000-rpm, high-temperature steam turbine, which
operates on hydrogen peroxide. The hydrogen-peroxide tanks also
supply the reaction-jet control system.

37. X–15 RESEARCH RESULTS
30
A second major subsystem is the air-conditioning unit, which
protects the pilot and instrumentation from the effects of heating
and also cools the auxiliary-power system. It operates from liquid
nitrogen, and, in addition to cooling, pressurizes the cockpit, instru-
ment compartment, pilot’s suit, hydraulic reservoir, and canopy seal.
One of the most complex and vital subsystems is the payload, the
research-instrument system. It was, of course, of utmost importance
to bring back a record of the temperatures and the aerodynamic
forces in this new environment, and the response of the structure to
them. This required installing thermocouples and probes and tub-
ing within the structure, as well as inserting them into the layer of
airflow around it. That necessitated cutting holes in the wings and
along the fuselage in locations that plagued the structural engineers,
and the installation had to be done while the airplane was being
built. By the time this work was completed, some 1400 pounds had
been added to the airplane’s weight. The research-instrument sys-
tem was perhaps the only one in which such a large weight would
be accepted, though reluctantly.
Throughout the design and construction, one goal for the X–15
was to make it as simple as possible, to use conventional design tech-
niques, and to use proven components wherever possible. Even
a cursory glance shows, however, that there is little conventional
about the X–15 or its systems. The many new concepts were the
products of necessity rather than desire. Newly conceived com-
ponents together with new materials and new processes have made
even simple systems become complex development projects. As a
result, a rigorous product-improvement-and-development program
is still underway five years after the first flight. Thus, from a 1956
aerodynamic design, a 1957 structural design, 1958 fabrication
techniques, and a 1959–64 development-test program, the X–15 has
evolved into an airplane in which updating and systems research
have been important factors.
The prime objective of the X–15 program has remained flight
research, however. By the time of the first flight, much had already
been learned about hypersonic flow by focusing the talents of many
men on X–15 problems. Many of the worries over flight above the
atmosphere had been dispelled. Yet hypersonic, exo-atmospheric,
and reentry-flight research was still a vague and obscure world.
Were the problems imagined or real? And what of those problems

38. DEVELOPING A CONCEPT 31
that man cannot foresee? The X–15 team was sure of only one
thing. The problems would come to light through probing the flight
corridor, until all the interactions among aerodynamics, structure,
stability, systems, and pilot control had been forced into view, and
the adequacy or inadequacy of man’s knowledge and capability
revealed.

40. 33
CHAPTER 4
Flight Research
THE HEART of an exploratory research program is planning.
For the X–15, it is nearly endless, and in a constant state of flux.
This work started with a feasibility study, which revealed that major
changes in flight-operations procedures from those of previous
research airplanes would be required. This grew into a program
of ever-increasing detail and variety to explore the many facets of
flight within the corridor as well as in the space-equivalent and
reentry regions.
With a performance capability of Mach 6 and 250 000 feet, the
X–15 had outgrown the type of operation that had suited the X–1
and X–2. The expanded requirements were evident in the B–52
launch airplane, pilot training, emergency-rescue facilities, emer-
gency-landing facilities, and in a facility to coordinate and control
each flight, as well as a radar and communications network. All
these had to be developed and integrated into an over-all plan
that would provide maximum support for the pilot on each flight.
Little wonder, therefore, that preparations for flight operations
started almost as early as the design studies began, in 1956. Work
was underway at various facilities of NACA, the Air Force, and
North American Aviation. Most of it was being done by the two
groups that would carry forward the flight-research program: the
NACA High Speed Flight Station and the Air Force Flight Test
Center, both at Edwards Air Force Base, California. These two
organizations had worked together in a spirit of cooperation and
friendly competition since the X–1 and D–558–I research programs
of 1947. They were experienced in the peculiarities of rocket-air-
plane operations and the techniques for exploring new aerodynamic
conditions in flight. To them, the X–15 was more than just the
newest of the X-series of research airplanes. The advanced nature
of the program, airplane, systems, and region of exploration would

41. X–15 RESEARCH RESULTS
34
require a supporting organization as large as the combined staff
needed for all previous rocket airplanes.
North American Aviation played a major role, of course, during
the initial phases of the flight program. Its demonstration and
de-bugging of the new airframe and systems comprised, in many
respects, the most arduous and frustrating period of flight operations.
The first year, in particular, was full of technical problems and heart-
break. One airplane split open on landing. Later, its hydrogen-
peroxide tank exploded, and its engine compartment was gutted by
fire on the ground. Another X–15 blew apart on the rocket test
stand. Flight research has never been painless, however, and these
setbacks were soon followed by success. Inevitably, the NAA flights
and the research program overlapped, since not only were two of the
three airplanes in operation but an interim rocket engine was in use
for the early flights. (The XLR–99 engine was delayed, and two
RMI XLR–11 rocket engines, having a total thrust of 16 000
pounds, were installed and flown for 30 flights of the X–15.)
In addition, exploratory research flights to determine practical oper-
ating limits merged with many of the detailed research flights, and
even with some flights carrying scientific experiments. Such flexi-
bility is normal, however, since flight research does not consist of
driving rigidly toward fixed goals.
The X–15 program progressed from flight to flight on foundations
laid upon freshly discovered aerodynamic and operational character-
istics. This research approach requires preflight analysis of all con-
straints on aerodynamic and stability-and-control characteristics, on
structural loads, and on aerodynamic-heating effects to determine
the boundary within which the flight can be made with confidence.
The constraints are regarded as critical limits, and the delicate
balance between adequacy and inadequacy can most easily be found
by approaching a limit yet never exceeding it.
Operational considerations require an answer to every question
of “What if this malfunctions?” before a pilot is faced with it, perhaps
critically, in flight. Often the success of a mission depends upon
the pilot’s ability to switch to alternate plans or alternate modes of
operation when a system or component fails. And flight research
requires a certain wariness for unanticipated problems and the inevi-
table fact that they become obvious only when a system or component
is exposed to them at a critical time. Yet some risk must be taken,

42. FLIGHT RESEARCH 35
for a too-conservative approach makes it almost impossible to attain
major goals in a practical length of time.
These factors have always been important to flight research, but
they were severely compounded in the case of the X–15. In investi-
gating the reentry maneuver and conditions of high aerodynamic
heating, the airplane is irrevocably committed to flight in regions
from which the pilot cannot back off in case he encounters an
unforeseen hazard. The complicating factor is that the load-carry-
ing ability of the heat-sink structure is not so closely associated with
specific speed-altitude-load conditions as it is in most other airplanes.
Instead, it depends largely upon the history of each flight up to the
time it encounters the particular condition. Therefore, it wasn’t at
all easy to predict margins of safety for the X–15’s structural temper-
atures in its initial high-heating flights. Moreover, since both air-
frame and systems were being continuously modified and updated
as a result of flight experience, many limiting conditions changed
during the program.
Thus, while an operational margin of safety has always governed
the program, rather diverse criteria have had to be used to define
that margin. Generally, each flight is a reasonable extrapolation
of previous experience to higher speed, altitude, temperature, angle-
of-attack, and acceleration, or to a lower level of stability. The
magnitude of the extrapolation depends on a comparison of flight
results, on wind-tunnel and theoretical analysis, on pilot comments,
and on other pertinent factors. The accuracy of aerodynamic data
determined in flight naturally has a bearing on flight planning. So
data-reduction and analysis are as important considerations as opera-
tional and piloting factors.
Through an intensive program of 26 flights in the 1960–62 period
(in addition to flights required for pilot-training or systems-check),
the X–15 probed flight to its design goals of Mach 6, 250 000-feet
altitude, and 1200o
F structural temperatures. This was very close
to the number of flights originally planned to reach those goals, but
the types of flight differed considerably from those of the initial plan.
Some deviations were made to explore a serious stability-and-control
problem found at high angle of attack. Another was made to ex-
plore high-heating conditions after thermal gradients greater than
expected had deformed the structure to a minor but potentially
dangerous extent.

43. X–15 RESEARCH RESULTS
36
The pace to push past the design goals was slower. Another year
and a half passed before the present maximum altitude of 354 200
feet was attained. Maximum temperature was raised to 1325o
F
in a flight to high-heating conditions at Mach 5 and low altittude.
A large measure of the success of the program has been due to a
research tool—the X–15 flight simulator—that was not available
when planning started, ten years ago. The flight simulator consists
of an extensive array of analog-computing equipment that simulates
the X–15 aerodynamic characteristics and computes aircraft motions.
Linked to the computer are exact duplicates of the X–15 cockpit,
instruments, and control system, including hydraulics and dummy
control surfaces.
Profile and pertinent details of a flight in which the X–15 achieved its design goals
in speed and altitude and came very close to that point in structural temperature.

44. FLIGHT RESEARCH 37
The X–15 flight simulator is somewhat like a Link trainer. But
its technology and complexity are as far advanced beyond those of
the Link trainer as the complexity of a modern high-speed digital
computer exceeds that of a desk-top adding machine. With the
simulator, both pilots and engineers can study flight conditions from
launch to the start of the landing maneuver. A flight is “flown”
from a cockpit that is exactly like that of the airplane. Only the
actual motions of pilot and airplane are missing.
Long before the first flight, X–15 pilots had become familiar with
the demands for precise control, especially during the first 85
seconds—the powered phase, which establishes conditions for the
entire flight. They had trained for the peculiarities of control above
the atmosphere with the jet reaction rockets. They had simulated
reentries at high angle of attack over and over again. The simula-
tor also gave them practice in the research maneuvers and timing
necessary to provide maximum data points for each costly flight.
They had practiced the many flight-plan variations that might be
demanded by malfunctions of rocket engine, subsystems, or pilot
display. They thus had developed alternate methods for completing
each mission, and had also developed alternate missions. Sometimes
the flight simulator proved its worth not so much by indicating exact
procedures as by giving the pilot a very clear appreciation of incor-
rect procedures.
Without this remarkable aid, the research program probably
would have progressed at a snail’s pace. Yet the flight simulator was
not ready-made at the start of the program. In fact, the complete
story of its technology is in large measure the story of how it grew
with the X–15 program. The potential of flight simulators for
aircraft development was just beginning to be appreciated at the
time of the X–15 design. Thus there was interest at the start in
using one to study X–15 piloting problems and control-system charac-
teristics. Early simulators were limited in scope, though, and con-
centrated upon control areas about which the least was known: the
exit condition, out-of-atmosphere flight, and reentry. Noteworthy
was the fact that angle of attack and sideslip were found to be primary
flight-control parameters, and hence would have to be included in
the pilot’s display. One of the chief early uses of the simulator was
to evaluate the final control-system hardware and to analyze effects
of component failures.

45. X–15 RESEARCH RESULTS
38
The initial simulations were expanded, and it soon was apparent
that the simulator had a new role, far more significant than at first
realized. This was in the area of flight support; namely pilot train-
ing (as already described), flight planning, and flight analysis. The
two last matters are closely interlocked, which insured that each step
in the program would be reasonable and practical. Pilot training
was also closely integrated, since often the margin of safety was in-
fluenced by a pilot’s confidence in the results from the flight simula-
tor. During the exploratory program, the capability of the simula-
tor to duplicate controllability at hypersonic speeds and high angle
of attack was an important factor in determining the magnitude of
each subsequent step up the flight corridor.
Even after 120 flights, pilots spend 8 to 10 hours in the simula-
tor before each 10–12-minute research flight.
The importance of the flight simulator today reflects the confidence
that pilots and research engineers have gained in simulation tech-
Before each 10–12-minute research mission, X–15 pilots train as long as 10 hours in
the electronic simulator at Edwards AF Base. Chief Research Pilot Walker is
sitting in its cockpit here. The simulator duplicates the X–15’s cockpit, instru-
ments, and control system, including hydraulics and dummy control surfaces, and
is nearly as long as the aircraft itself.

46. FLIGHT RESEARCH 39
niques. This confidence was lacking at the start of the program,
since the simulator basically provides instrument flight without
motion cues, conditions not always amenable to extrapolation to
flight. However, much has been learned about what can and can-
not be established on a flight simulator, so that even critical control
regions are now approached in flight with much confidence.
The application of the X–15 simulation techniques to other pro-
grams has accelerated flight-simulation studies throughout the aero-
space industry. Interestingly, this is one of the research results not
foreseen.
Navy’s Centrifuge Valuable Aid
A notable contribution to flight simulation was also made by the
Navy, theretofore a rather silent partner in the X–15 program. The
Aviation Medical Acceleration Laboratory at the Naval Air Devel-
opment Center, Johnsville, Pa., has a huge centrifuge, capable of
carrying a pilot in a simulated cockpit. The cockpit is contained in
a gondola, which can be rotated in two axes. It is mounted at the
end of a 50-foot arm. By proper and continuous control of the two
axes in combination with rotation of the arm, the forces from high-G
flight can be imposed on the pilot. This centrifuge was an ideal
tool with which to explore the powered and reentry phases of X–15
flights.
Another significant aspect of the NADC centrifuge soon became
apparent. Previously, the gondola had been driven along a pro-
gramed G pattern, not influenced by the pilot; he was, in effect, a
passenger. But in flight an X–15 pilot not only would have to with-
stand high G forces but maintain precise control while being squashed
down in his seat or forced backward or forward. lt was important
to find out how well he could maintain control, especially during
marginal conditions, such as a stability-augmentation failure during
reentry. The latter would superimpose dynamic acceleration forces
from aircraft oscillations on already severe pullout G’s. There were
no guidelines for defining the degree of control to be expected from a
pilot undergoing such jostling.
To study this phase, the NADC centrifuge was linked to an elec-
tronic computer, similar to the one used with the X–15 flight simu-
lator, and the pilot’s controls. The computer output drives the
centrifuge in such a manner that the pilot experiences a convincing

47. X–15 RESEARCH RESULTS
40
approximation of the linear acceleration he would feel while flying
the X–15 if he made the same control motions. (The angular
accelerations may be unlike those of flight, but normally they are of
secondary importance.) This type of closed-loop hookup (pilot
control to computer to centrifuge) had never been attempted before.
It was a far more complex problem than developing the electronics
for the immobile flight simulator.
With this centrifuge technique, pilots “flew” about 400 reentries
before the first X–15 flight. The G conditions on most of these
simulated reentries were more severe than those experienced later
in actual flights. The simulation contributed materially to the de-
velopment and verification of the pilot’s restraint-support system,
instrument display, and side-located controller. The X–15 work
proved that, with proper provision, a pilot could control to high
acceleration levels.
Aside from its benefit to the X–15 program, the new centrifuge
technique led to fresh research into pilot control of aircraft-space-
craft. The Aviation Medical Acceleration Laboratory was soon
deluged with requests to make closed-loop dynamic flight simulations,
particularly for proposed space vehicles. Many of these studies have
now been completed. They have shown that pilot-astronaut con-
trol is possible to 12–15 G’s. This research will pay off in the next
generation of manned space vehicles. The X–15 closed-loop pro-
gram was also the forerunner of centrifuges that NASA has built for
its Ames Research Center and Manned Spacecraft Center.
In addition to hundreds of hours of training with the flight simu-
lator and the NADC centrifuge, the X–15 pilots have also trained in
special jet aircraft. These aircraft were used for limited explora-
tions of some of the new flight conditions. For example, an ex-
ploratory evaluation of the side controller was made as early as 1956
in a T–33 trainer, and later in an F–107 experimental aircraft.
Other tests were made of reaction jet controls, and the reentry
maneuver was explored with two special test aircraft that were in
effect airborne flight simulators. One of the earliest programs, still
in use, is X–15 approach-and-landing training in an F–104 fighter.
This practice, which involves deliberately inducing as much drag
as possible, has been especially important in maintaining pilot pro-
ficiency in landing, since for any single pilot there are often long
intervals between X–15 flights.

48. FLIGHT RESEARCH 41
Many flight tests were made to integrate the X–15 with the B–52
launch-airplane operation. The air-launch technique had been
proven, of course, with previous rocket airplanes. The concept has
grown, however, from a simple method for carrying the research air-
craft to high initial altitude, to an integral part of the research-air-
craft operation. For the X–15, the air-launch operation has be-
come in effect the launching of a two-stage aerospace vehicle, utiliz-
ing a recoverable first-stage booster capable of launching the second
stage at an altitude of 45 000 feet and a speed of 550 mph. As
with any two-stage vehicle, there are mutual interferences. They
have required, among other things, stiffening of the X–15 tail struc-
ture to withstand pressure fluctuations from the airflow around the
B–52 and from the jet-engine noise.
Several of the X–15 systems operate from power and supply
sources within the B–52 until shortly before launch; namely, breath-
ing oxygen, electrical power, nitrogen gas, and liquid oxygen.
These supplies are controlled by a launch crewman in the B–52, who
also monitors and aligns pertinent X–15 instrumentation and electri-
cal equipment. In coordination with the X–15 pilot, he helps make
a complete pre-launch check of the latter aircraft’s systems. Since
this is made in a true flight environment, the procedure has helped
importantly to assure satisfactory flight operations. The mission
can be recalled if a malfunction or irregularity occurs prior to second-
stage launch. These check-out procedures are also important to
B–52 crew safety, since the explosive potential of the volatile propel-
lants aboard the X–15 is such that the B–52 crew has little protection
in its .040-inch-thick aluminum “blockhouse.”
The launch is a relatively straightforward free-fall maneuver, but
it was the subject of early study and concern. Extensive wind-
tunnel tests were made to examine X–15 launch motions and develop
techniques to insure clean separation from the B–52.
The X–15 required a major change in flight operations from
those of previous rocket airplanes, which had operated in the near
vicinity of Rogers Dry Lake, at Edwards. With a Mach 6 capa-
bility, the X–15 had outgrown a one-base operation, since it may
cover a ground track of 300 miles on each flight. The primary
landing site is at Edwards, which requires launching at varied dis-
tances away from the home base, depending on the specific flight
mission and its required range. A complicating factor in flight

49. X–15 RESEARCH RESULTS
42
operations is that the launch must be made near an emergency land-
ing site, and other emergency landing sites must be within gliding
distance as the craft progresses toward home base, for use in the event
of engine failure.
Fortunately, the California-Nevada desert region is an ideal
location for such requirements, because of many flat, barren land
areas, formed by ancient lakes that are now dry and hard-packed.
Ten dry lakes, spaced 30 to 50 miles apart, have been designated for
X–15 use, five as emergency landing sites near launch location, five
as emergency landing sites down-range. The X–15 pilots are
thoroughly familiar with the approach procedures for all emergency
landing sites.
Because of wide variations in the research maneuvers, successive
flights may be made along widely separated ground tracks. The
track will normally pass within range of two or three emergency
sites. The desired research maneuvers often must be altered to make
sure that the flight path passes near emergency landing sites. These
This drawing shows the flight paths of two typical research missions of the X–15.
Radar stations at Beatty and Ely, Nev., and at home base track each flight from
takeoff, attached to a B–52 drop plane, to landing. Launch always occurs near
one of the many dry lakes in the region, some of which are indicated here.

50. FLIGHT RESEARCH 43
procedures are studied on the flight simulator, and pilots predeter-
mine alternate sites and the techniques to reach them for each flight.
On four occasions, rocket-engine malfunctions have necessitated
landing at an emergency site.
Emergency ground-support teams, fire trucks, and rescue equip-
ment are available at all sites. Airborne emergency teams, consist-
ing of helicopters with a rescue team and a C–130 cargo airplane
with a pararescue team, are also positioned along the track.
An important adjunct to mission success has been the extensive
support the X–15 pilot receives during a flight from the many people
“looking over his shoulder,” both in the air and on the ground. On
hand during a flight are chase aircraft, which accompany the B–52
to the launch point. Although these are soon left far behind after
X–15 launch, other chase planes are located along the intended track
to pick up the X–15 as it nears the primary or alternate emergency
landing sites.
Coordination and control of the farflung operation are carried out
from a command post at the NASA Flight Research Center. Into
it comes information pertinent to the X–15’s geographic location,
performance, and systems status, and the status of the B–52, chase
planes, and ground-support teams. Responsibility for the coordina-
tion of this information, as well as for the complete mission, rests
with a flight controller. This function is carried out either by one
of the X–15 pilots or by some other experienced research pilot. The
flight controller is in communication with the X–15 pilot at all times,
to provide aid, since he has far more information available to him
than the pilot has. This information is provided by a team of
specialists who monitor telemetry signals from the airplane. One of
the primary functions of the flight controller is to monitor the X–15’s
geographic position in relation to the amount of energy it will need
to reach an intended landing site. The flight controller also provides
navigation information to help the X–15 pilot reach any desired site.
The flight controller’s capability to monitor the complete operation
is provided by a radar-telemetry-communications network that ex-
tends 400 miles, from Edwards to Wendover, Utah. Ground stations
are located at Edwards; Beatty, Nevada; and Ely, Nevada. Each
station is an independent unit, though all stations are interconnected
by telephone lines or microwave-relay stations. This network is
another joint USAF–NASA facility. Like most other features of

51. X–15 RESEARCH RESULTS
44
the program, the range has been updated to provide additional
flexibility, accuracy, and/or reliability.
Another integral part of a flight-research program is extensive and
detailed measurements of aircraft behavior. These measurements
enable X–15 pilots to approach critical conditions with confidence,
and also provide data to uncover unforeseen problems. However,
determining suitable instrumentation is not an exact science. In
many cases, although the airplane seemed to be overinstrumented
during design, it was found to be underinstrumented in specific areas
during the flight program. In addition, many compromises had to
be made between the amount of instrumentation for research meas-
urements and that for systems monitoring. Other compromises
were necessary for measuring and recording techniques. A vast array
of gauges, transducers, thermocouples, potentiometers, and gyros is
required to measure the response of the X–15 to its environment.
Because of the difficulty of measuring pressures accurately in the
near-vacuum conditions of high-altitude flight, an alternate method
for measuring velocity and altitude had to be developed. The system
uses a missile-type inertial-reference system, with integrating accel-
erometers to determine speed, altitude, and vertical velocity. The
system also measures airplane roll, pitch, and yaw angle relative to
the Earth, to indicate aircraft attitude to the pilot. Alignment and
stabilization are accomplished during the climb to launch altitude
by means of equipment within the B–52.
Another system development was required for measuring angles of
attack and yaw. Although flight measurements of these quantities
had always been important for analysis of aerodynamic data, they
took on added significance for the X–15 when early simulator studies
showed that they would be required as primary pilot-control param-
eters during much of a flight. Rather severe requirements were
placed on the system, since it would have to measure airflow angles
at air temperatures to 2500o
F and have satisfactory response for
very low as well as high air pressures. The system consists of a
sphere, 6½ inches in diameter, mounted at the apex of the airplane
nose. This sensor is rotated by a servo system to align pressure
orifices on the sphere with the airflow. The system has been highly
successful for the precise control that the X–15 requires.
A most important contribution to mission success is the “blood,
sweat, and tears” of the men who work to get the X–15 off the

52. FLIGHT RESEARCH 45
ground. An unsung effort, averaging 30 days in duration, is re-
quired to prepare and check-out the airplane and systems for every
flight. Many of the systems and subsystems were taking a larger
than normal step into unknown areas. Inevitable compromises
during design and construction resulted in an extensive development
effort for many components and subsystems, as part of the flight-
research program. A rigorous program of product improvement
and updating of systems has continued throughout flight operations.
While this work ultimately forced a somewhat slower pace upon the
program, its results are found in the remarkably successful record of
safe flight operations and in-flight reliability.
The flight achievements, of course, are the payoff for the metic-
ulous preparations that have gone on for the past 10 years. With-
out this vast support, the pilots might have taken too large a step
into new flight regimes. While many problems were encountered,
they have been surmounted, some as a result of pilot training, others
as a result of measurements of the response of the airplane to the
new flight environment.
Just as each X–15 flight leaves a few less unknowns for succeed-
ing flights, so will the X–15 program leave a few less unknowns for
succeeding airplanes. By exploring the limits of piloted flight within
the corridor as well as above it, man has expanded his knowledge in
many fields. The real significance of the four miles of data from
each flight came from tedious analysis of the response, which pro-
vided some insight into basic forces. Sometimes an examination
of gross effects sufficed, but more often it required a penetrating look
into the very core of aerodynamic flow. From this has come the
first detailed picture of airflow around an airplane at hypersonic
speeds.

54. 47
CHAPTER 5
Aerodynamic Characteristics of
Supersonic-Hypersonic Flight
O
UR DEPTH OF UNDERSTANDING of how we fly has come
from study of the mechanics of flight and the theory of airflow.
This comprises the science of aerodynamics, which has its roots in
the study of fluid mechanics and concerns all the forces acting on
an airplane as a result of its motion through the air. When an air-
plane passes through the atmosphere, the air molecules behave like
a fluid, flowing around the wings and fuselage, tending to stick to
the surface and be dragged along behind, and, under certain con-
ditions, being compressed. The pressure from this flow exerts the
well-known lift and drag forces, and the less familiar stabilizing
forces.
Airflow shows an amazing variety of characteristics, which have
been the subject of intensive theoretical analysis and study in wind
tunnels. At slow speeds, the pressures an airplane generates as it
moves through the air are small relative to the ambient atmospheric
pressure. The balance between these two pressures establishes the
boundaries of the aerodynamic flight corridor. The pressure pro-
duced by motion, called dynamic pressure, increases as the square
of velocity. At Mach 1.2, the dynamic pressure is equal to the
atmospheric pressure; at Mach 6, it is 25 times greater. This in-
crease in dynamic pressure permits sustained flight at high altitude,
where the atmospheric pressure is extremely low, provided the speed
is high enough.
Pressure forces are also affected by changes in airflow, from its
elastic and viscous characteristics as it flows around an aircraft.
Drastic changes in flow, as previously noted, are encountered in flight
to high speeds. At 4000 mph, the airflow bears little resemblance to
that at 400 mph. It will, in fact, have gone through four regions:
subsonic, transonic, supersonic, and hypersonic.

55. X–15 RESEARCH RESULTS
48
These dramatic photographs of free-flight models of the X–15 being fired into a
wind tunnel vividly detail the shock-wave patterns for airflow at Mach 3.5 (above)
and at Mach 6.

56. AERODYNAMICS OF SUPERSONIC-HYPERSONIC FLIGHT 49
The major consequence of flight to high speed is the effect on
airflow, because of the elasticity (compression and expansion) of
air. At the slowest speeds, subsonic, the effects are not pronounced.
As airflow velocities increase, the air becomes compressed, and pres-
sure begins to pile up ahead of each part of the aircraft, until finally
distinct pressure waves, or shock waves, form. The transonic air-
flow region is where shock waves first appear on an aircraft, though
these shocks may be only local in nature. It is a region of mixed and
erratic flow between subsonic and supersonic flow, which causes
abrupt changes in lift and drag forces and airplane stability. As
speed is further increased, local regions of subsonic flow disappear,
and the flow is everywhere supersonic. The air has become further
compressed. The shock waves are now distinct and trail aft in the
form of a wedge, or cone, behind any object that interferes with the
airstream. While a shock wave is normally less than .001-inch
thick, the air undergoes large changes in pressure, density, and tem-
perature across this minute boundary. These effects are far-reach-
ing, even extending to the ground in the form of sonic booms. Aero-
dynamic theory has been developed that enables the characteristics
of these shock waves to be precisely calculated.
At higher supersonic speeds, the shock waves continue to increase
in strength, bending back to form an acute angle with the aircraft
surfaces. The equations of supersonic flow at this point no longer
apply, and many interactions between shock waves and flow field are
evidenced. One major effect is a loss of lifting effectiveness of the
wings and tail surfaces, because the shock waves attenuate the aero-
dynamic forces. Of more significance, the friction of the air flow-
ing along any surface raises air temperature to many times that of
the surrounding atmosphere. Airflow is now in the hypersonic-
flow region, and the science of thermodynamics is added to aerody-
namics. Though not exactly defined, it is generally accepted as
applying to speeds above about Mach 5. It is an area of multiple
shock waves and interference effects. The difficulty for the aero-
dynamicist arises from trying to understand the effects of flow that
is discontinuous at each shock wave. Each new geometric shape
calls for reorganization of theory.
By optimizing the shape, size, and relative locations of wing, tail,
and fuselage, an airplane is made highly efficient for one flow
region. But that particular configuration may have many adverse

57. X–15 RESEARCH RESULTS
50
interference effects when airflow enters a new flight regime. Many
compromises are necessary to achieve one configuration that is satis-
factory from subsonic to hypersonic speeds.
Facing Major Gaps in Knowledge
At the time the X–15 was designed, theory and empirical data
(much of it from previous research airplanes) provided a good
understanding of the mechanics of airflow for speeds to about Mach
3. But there were major gaps in aerodynamic knowledge above
this speed. Some of these gaps were bridged by wind-tunnel tests
of scale models of the X–15. However, although models of the X–
15 were tested in many supersonic and hypersonic wind tunnels,
they were of very small scale—1/15 or 1/50—and no verification had
been made of the results from small-scale models for flight at hyper-
sonic speeds. Moreover, wind tunnels approximate flow conditions
rather than exactly duplicating them. Hence, a valuable part of the
X–15 program would be to verify or modify the picture of hypersonic
flow derived from these experimental techniques and from theoretical
analyses.
Over the years, various analytical techniques have been developed
by which basic aerodynamic characteristics can be extracted from
flight measurements of airplane response. In general, it was found
that these techniques could be extended to the X–15’s ranges of speed
and angle of attack. However, since most X–15 maneuvers are of
a transient nature, the evaluation of dynamic motions was aided con-
siderably by using the flight simulator to “match” the actual flight
maneuvers. New techniques were required for the analysis of aero-
dynamic heating, however. Since the thermocouples provide only a
measure of the response of the structure, techniques were developed
on a digital computer to determine heat flow from the air into the
structure.
Details of Hypersonic Flow Emerge
From these analyses, the details of hypersonic flow began to unfold.
The results confirmed many of its nonlinear characteristics. The
data also confirmed another peculiar trend of hypersonic flight: the
reduced importance of the wings for lift. At Mach 6 and 25o
angle
of attack, the large fuselage and side fairings on the X–15 contribute